The present invention relates generally to glow plugs for diesel engines and, in particular, to glow plugs that are capable of measuring the ionization current inside the engine combustion chamber.
FIG. 3 of European published patent application number EP-A-0989370 discloses a glow plug provided with a tubular metal body and with a metal sheath electrically insulated from the tubular body. A heating resistor is housed inside the sheath and has one of its ends connected to the sheath and the other connected a first electrical terminal. The glow plug illustrated in EP-A-0989370 comprises a second terminal electrically connected to the sheath. Electrical insulation between the sheath and the tubular body is obtained by means of a pair of rings made of ceramic material and set at the opposite ends of the tubular body. The second electrical terminal consists of an electrical wire provided with an insulating coating, which is welded to the end edge of the sheath and is set inside the tubular body.
The solution described in EP-A-0989370 presents a number of drawbacks. The solution according to the prior art requires a very long sheath which reaches as far as the ceramic ring set at the end of the tubular body opposite to the end from which the sheath protrudes. The electrical wire constituting the second terminal needs to be welded in order to create the electrical connection with the internal edge of the sheath, and this weld involves problems of resistance over time and affords poor guarantees of stability of the electrical connection.
In the case of the embodiment illustrated in FIGS. 4 and 5 of EP-A-0989370 the use is envisaged of three electrical contacts, which increase the cost of the finished product. In this variant, the heating resistor is not fixed to the sheath, and there are problems of vertical alignment and centring of the heating element with respect to the sheath.
A general object of the present invention is to provide a glow plug of the type indicated above that makes it possible to overcome the drawbacks referred to previously.
According to the present invention, there is provided a glow plug for diesel engines that includes a metal tubular body and a metal sheath carried by the tubular body. The sheath has an end portion projecting from the tubular body and is electrically insulated from the tubular body. The glow plug includes first and second electrical terminals and a heating resistor electrically connected to the first terminal and to the end portion of the sheath. The first terminal has the shape of a metal rod extending through the tubular body and has an end inserted inside the sheath. The heating resistor is also set inside the sheath. The second terminal is electrically connected to the sheath and is disposed coaxially with respect to the first terminal.